We have approached the plenum multiple ways with multiple materials, including all surfaces. Just not a really noticeable difference there.
We have done header wraps, coatings, etc. Probably the biggest benefit is underhood temps, which help shock absorbers, especially with remote reservoirs. We see minimal power difference, but protecting other components is important. Sometimes with our race tuning, the headers can get to glowing yellow or hotter, especially in warmup.